It is well known in the art that superconductors, such as high-temperature superconductors (“HTSs”), act as powerful analogs to permanent magnets, and electrical machines made with HTSs have many features of brushless motors and generators that use permanent magnets. It is further known in the art that magnetic flux that is trapped in HTSs may be used to achieve unprecedentedly high specific power levels in electrical motors and generators as opposed to permanent magnets. As a point of comparison, the magnetization of the best permanent magnets is about 1 to 1.5 T. On the other hand, trapped magnetic flux in HTSs has been shown to have a world record magnetization of about 17.6 T, which is over an order of magnitude higher than permanent magnets.
One of the significant challenges with use of HTSs pertains to how to effectively charge the HTSs in-situ. Prior art attempts to magnetize HTSs in-situ have resulted in trapped magnetic fields that were not much larger than what can be achieved with the use of permanent magnets. Thus, in order to achieve the world record trapped magnetic field as described above using prior art methods, an HI'S was charged by a very large superconducting magnet and the HTS was contained in an isolated laboratory environment rather than in a machine. Also, in order to obtain the world record magnetization using prior art methods, the external magnetic field was applied to the I-ITS and then it was field cooled (FC). This method is contrary to the in-situ charging method described below with respect to the present disclosure, which typically involves zero-field cooling (ZFC), wherein the HTS is first cooled and then a magnetic field is applied. Typically, to obtain the same amount of trapped flux, the ZFC procedure requires twice the applied magnetic field than does a FC procedure.
After magnetizing the HTS with a large superconducting magnet in a laboratory environment according to the prior art method just described, the bulk HTS is then assembled in a machine. However, this entire assembly process has to be done cold, meaning that the assembly has to be conducted in a cold room, and very likely in a vacuum environment as most gases would condense at the cryogenic temperature required. In addition, this prior art method requires the use of strong robotic manipulators to complete the machine assembly because of the large magnetic forces associated with energized magnets. The assembled machine is then removed from the cold room and the outer enclosure of the machine is allowed to warm to ambient temperature, but the HTSs have to remain at cryogenic temperatures in order to not lose their magnetization. While such prior art method of assembly is feasible, the paradigm of keeping a motor or generator cold over its operating life has not been adopted by industry consumers of high specific-power machines. Rather, consumers would prefer to magnetize HTSs in-situ, so that the machine can be assembled with the HTSs unmagnetized and at ambient temperature.
A further drawback of the prior art methods for putting trapped magnetic flux in HTSs is that the diameter of the magnet needed to produce the required field is bigger than the diameter of the HTS, particularly because the magnetizing flux enters the HTS from its outer perimeter surface. From an in-situ standpoint, this is a significant disadvantage because such a large power supply is needed to energize the magnet.
It is, therefore, desirable, to have an apparatus and method for efficiently charging bulk HTSs in-situ.